Porosity of activated carbons

Many investigators have used nitrogen adsorption at 77 K to study the porosity of activated carbons (e.g. see de Vooys, 1983 Femandez-Colinas et al., 1989a Rodriguez-Reinoso et al, 1989 Sing, 1989,1995 Freeman and Sing, 1991 Bradley and Rand, 1995). 

Other investigations of the porosity of activated carbon cloth explored the effects of impregnation with certain transition metal salts and oxo-complexes (Freeman etal., 1989) and of activation in ammonia (Tomlinson etai, 1993). In the latter case it was found that ammonia reacts with the viscose rayon chars to form various nitrogen-containing microporous products. An early study by Barton and Koresh (1983) had also demonstrated that the pore structure of carbon cloth and its affinity for water vapour can be modified by HN03 activation. 

Gonzalez J.C.,.Gonzalez M.T, Molina-Sabio M. Rodriguez-Reinoso F. (1995) Porosity of activated carbons prepared from different lignocellulosic materials. Carbon, 33,1175,... 

In the present work, positron annihilation lifetime spectroscopy has been applied to characterize the porosity of activated carbons fibers. These materials are essentially microporous [16], with slit shaped pores and with a homogeneous pore size distribution. Because of that, they seem to be the most appropriate materials to analyze the application of PALS technique to the characterization of porous carbon materials. 

The units of monolayer coverage could just as well be mmolg in, for example, microporosity. It is only because the units of are easier to translate into a physical picture that they have stayed within the adsorption literature. Within the porosities of activated carbon, the surface is a boundary condition which limits the proximity of other atoms (adsorbed). It is a limit of electron density and as such will be curved. Figures 1.1 and 1.2 (Chapter 1) suggest this. 

The contents of this chapter summarize the several methodologies used to characterize the porosity of activated carbon. The isotherms of the N2 (77 K), CO2 (273 K), H2O (298 K), making use of DR and BET equations, together with a-plots, in association with enthalpies of immersion, characterize porosity in activated carbon. Equilibria data are complemented by the kinetic data from breakthrough curves. 

Or water vapor. These positions are the basal plane, or at armchair edges, or at zigzag edges. All positions have different reactivities . The relevance here is that the defective micro-graphene layers which constitute the surfaces of the porosity of activated carbons may well exhibit some form of reaction anisotropy based on these considerations. 

Benaddi H, Bandosz TJ, Jagiello J, Schwarz JA, Rouzaud, Legras D, Beguin F. Surface functionality and porosity of activated carbons obtained from chemical activation of wood. Carbon 2000 38(5) 669-674. 

Figure 3 shows the influence of parent coal preoxidation on the porosity of activated carbons obtained when two different flow rates of CO2 are used in the activation step. In both cases an increase in porosity is observed with coal preoxidation, which is more important for lower flow rate of CO2. 

Figure 3 includes plots of cumulative pore volume deduced from mercury porosimetry for carbon of series B, activated in CO and steam by the uncatalyzed and catalyzed activation process. Uncatalyzed activation with CO only develops the macroporosity especially in the pore range 70-1500 nm. In the case of steam activation the porosity development occurs in pore sizes below 300 nm, in agreement with the results obtained from N, adsorption. Similar trends are found with carbons of series A. These finding are in agreement with data published previously where the porosity of activated carbons prepared by CO, and steam activation of carbonized plum and olive stones were compared (ref. 7). 

In this chapter, we present in some detail gas adsorption techniques, by reviewing the adsorption theory and the analysis methods, and present examples of assessment of PSDs with different methods. Some examples will show the limitations of this technique. Moreover, we also focus on the use of SAXS technique for the characterization of porous solids, including examples of SAXS and microbeam small-angle x-ray scattering (pSAXS) applications to the characterization of activated carbon fibers (ACFs). We remark the importance of combining different techniques to get a complete characterization, especially when not accessible porosity exists. 

The high porosity that results from activation increases the area for adsorption. One gram of char can produce about 1000 m of adsorption area. After activation, the char is further processed into three types of finished product powdered form called powdered activated carbon (PAC), the granular form called granular activated carbon (GAC), and activated carbon fiber (ACF). PAC is normally less than 200 mesh GAC is normally greater than 0.1 mm in diameter. ACF is a fibrous form of activated carbon. Figure 8.7 shows a schematic of the transformation of raw carbon to activated carbon, indicating the increase in surface area. 

The porous textural characterization of activated carbons is a very important subject due to the growing interest in the preparation of materials with well-defined pore structures and high adsorption capacities. Porosity characterization is an essential task to foresee their behavior in a given use and requires a combination of different techniques. Gas adsorption techniques constitute the most common approach to the characterization of the pore structure of porous materials. However these techniques have some limitations. 

These examples show that pSAXS experiments are an easy way to analyze the isotropy of the porosity in activated carbon fibers and to observe the development of porosity with the activation process. 

Figure 1 shows the N2 adsorption isotherms at 77K of the series of activated carbon fibers obtained from CO2 and steam activation. The N2 adsorption isotherm corresponding to the original fiber (i.e., without activation) is not shown because this sample presents a very narrow microporosity, not accessible to N2 at 77K [18,19], The kinetics of N2 adsorption is extremely slow, and very long times should be necessary to reach the equilibrium in each point of the isotherm. However, in this narrow porosity CO2 adsorption occurs [18-20],... 

Lozano-Castello D., Lillo-Rodenas M.A., Cazorla-Amoros D., Linares-Solano A., Preparation of activated carbons from Spanish anthracite. Carbon 2001 39 741-749. Weng T.C., Teng FI., Characterization of high porosity carbon electrodes derived from mesophase pitch for electric double-layer capacitors, J. Electrochem. Soc. 2001 148 368-373. 

---